This invention relates to a form-fill-seal packaging machine for forming bags while concurrently filling and sealing them with articles to be packaged.
In general, a packaging machine of this type is structured so as to bend an elongated web of thermoplastic bag-making material (herein referred to as the “film”) into a tubular form around a chute through which articles are dropped to be packaged, to longitudinally seal its mutually overlapping side edges by means of a longitudinal sealer and then to seal the tubularly formed film transversely to produce a packaged product.
The aforementioned process of longitudinal sealing is usually effected by causing a heated heater block to intermittently come into contact with the chute so as to sandwich and compress the overlapped edge parts of the film between the block and the outer front surface of the chute while the film is being transported longitudinally (the intermittent mode of film transportation) or by leaving a heated metallic heater belt in contact with the chute such that the overlapped edge parts of the film is continuously pressed against each other between the block and the chute (the continuous mode of film transportation). In what follows, such a heater block and a heater belt will be both referred to as the “heater” and discussed together for convenience.
In either mode of transportation described above, the heater is adapted to move between a sealing position where the heater comes into contact with the chute to effect the longitudinal sealing and a retracted position to which the heater is moved away from the chute. In the intermittent mode of transportation, the heater is moved to a retracted position while the film is being transported. In the continuous mode of transportation, on the other hand, the heater is moved to the retracted position when, for example, a trouble has occurred and the transportation of the film is interrupted temporarily in order to prevent the overheating of the film.
While the heater is at the sealing position, it presses the film against the chute with a specified pressure for the thermal sealing. This compressive pressure is usually supplied by the biasing force of a spring, and it has been the practice to adjust its compressive force. It sometimes happens when the chute is attached to the main frame of the packaging machine, however, that the chute is not exactly at the intended position, depending on the manner in which the attachment is effected, or that the relative position changes or the shape of the heater changes after many years of use. In such a situation, the sealing position of the heater may change and the compressive force by the spring may also change. Thus, the compressive force of the spring had to be adjusted as the sealing position of the heater changed.
Manual adjustment of the spring could not be done very accurately. Moreover, the recent trend is that the compressive force of the spring must be adjusted not only according to the film characteristics such as its thickness and material but also according to the method of sealing such as the so-called lap sealing and fin sealing and also according to the operational capabilities and conditions. In other words, fine adjustments of the compressive force of the spring according to many factors are now a commonly required procedure.
As explained above, furthermore, the heater must generally be moved between a sealing position and a retracted position. A quick response is required in this motion such that the heater can move between these positions as instantaneously as possible. This means that a fairly large force is necessary for the motion of the heater and further that a different kind of improved control is necessary. Thus, there is now a new problem of how to control these two kinds of forces on the single heater.